1. Field of the Invention
The present invention relates to a thread feeding device for feeding/controlling threads, necessary for one seam, to two or more needles and/or loopers of a sewing machine and, more particularly, to a thread feeding device capable of adjusting the feeds of the threads by a single operation.
2. Description of the Related Art
In a sewing machine equipped with pluralities of needles and loopers, such as an overlock sewing machine, generally speaking, the feeds of threads to the needles and the loopers are effected by thread feeding mechanisms of the let-off and rotary types. In the let-off type, the threads are let off by the actions of the needle take-ups and the loopers, and the amounts necessary for one seam are adjusted by a thread tensioning device such as tension discs disposed in the individual feed paths of the threads.
In a three-thread overlock sewing machine, for example, the individual threads including the needle threads and the upper and lower looper threads let off their bobbins are fed to the needles and the upper and lower loopers, respectively, through the thread tension discs and thread guides disposed on the individual face plates. Then, the needles and the loopers act to tension and slacken the threads between the seam already formed by one sewing action and the thread tension discs, and the threads are pulled by the needle thread take-ups. During this action, the tension discs hold the individual threads under suitable tension to form neat seams. In this case, the tension devices each clamp a thread between two discs and push it by a spring to tense the thread. If the type of the thread is changed, however, the thickness and sliding resistance of the thread will change, so that the needle thread take-ups must be adjusted again. In the overlock sewing machine, for example, the needle thread and the upper or lower looper thread intersect on the upper or lower face of a cloth, and the upper and lower looper threads intersect at the side edge of the cut cloth to form loops at the cloth edge portion. Since, however, the seams (as shown in FIG. 9) are governed by the tensions of the individual threads, if the adjustment of the three threads is inferior, the intersections between the upper looper thread and the lower looper thread may appear on the upper side of the cloth, or one of the threads may be lashed so that neat seams cannot be formed.
The rotary type, on the other hand, is exemplified by a device (as disclosed in Japanese Patent Publication No. 4137/1961 or Japanese Utility Model Laid-Open No. 45754/1974), in which the threads are clamped by two thread tension discs which are rotated to feed the threads and the threads may be tensioned by forcing frictional members against the discs. In another rotary type device (as disclosed in Japanese Patent Laid-Open No. 101939/1979), the threads are wound on a rotor and frictional members are forced onto the rotor so that the rotor may be rotated to feed and tension the threads.
In the thread tensing device of the rotary type, however, the threads are fed in given lengths without regard for their type. Thus, the thread tensions do not vary even if the thread type is changed, so that the seams obtained have no irregularity. However, the means for winding the threads between the two thread tension discs, without any slippage, is complicated.
To eliminate these difficulties, there has been proposed a device (as disclosed in Japanese Patent Laid-Open No. 58592/1982), in which one of two rollers clamping a thread is controlled by an arithmetic control unit so that a predetermined amount of thread may be let off. Another sewing thread feeding device (as disclosed in Japanese Patent Laid-Open No. 45088/1990), is constructed as shown in FIG. 8, and includes: a let-off mechanism (i.e., a roller with an encoder) 101 for clamping and letting off a plurality of threads: electromagnetic thread grippers 102 disposed upstream of the let-off mechanism 101, each individually operating on one of the threads; and an electromagnetic common thread gripper 103 disposed downstream, so that the feeds of threads may be electrically controlled. With this latter thread feeding device, the threads are forcibly let off by the encoder roller 101, and the upstream thread grippers 102 are opened or closed, according to the feeds required of the individual threads, whereas the downstream thread gripper 103 is opened during the thread pulling actions of the thread take-ups and the upper and lower loopers, so that the threads may be individually fed in predetermined amounts.
Since the threads are forcibly fed by the rollers or the encoder rollers, by the above-described thread feeding devices, the threads can be accurately let off without any fear of tension fluctuation. However, because the thread grippers are electrically driven, there arises the problem that the device becomes complex and expensive. Especially in the case where the thread feeds are to be adjusted, the drive of the rollers or the opening/closing time periods of the thread grippers must be controlled by an arithmetic control unit, such as a computer, so that the cost of the device includes that of a microcomputer. Moreover, a plurality of threads are let off by a common let-off mechanism. Thus, in the case where different kinds of threads, e.g., a thick ribbon-shaped thread and an ordinary sewing thread, are combined to form a loop, a sufficient letting-off force may not be applied to the thinner thread although the thicker thread can be let off.